refactor: introduce Client type, remove TUI from library, eliminate init() side effects

[zeroshade]

Summary

This branch refactors the dbc package from a package-global, init()-heavy design to a clean library API:

• New Client type with functional options (WithHTTPClient, WithRegistries, WithBaseURL, WithUserAgent, WithCredential, WithAuthFromFilesystem) replacing package-level singletons
• TUI dependencies removed from root dbc and config packages — import "github.com/columnar-tech/dbc" now pulls zero bubbletea/lipgloss/bubbles transitive deps; widgets moved to cmd/dbc
• init() side effects eliminated — no disk writes, no http.DefaultClient mutation at import time; lazy sync.Once-based setup only triggered on first network call
• Old APIs deprecated (GetDriverList, DefaultClient) with forwarding shims for backward compatibility; CLI migrated to use Client internally
• 27 iterative fix commits addressing resource leaks, nil panics, race conditions, double-closes, and request-reuse violations surfaced by automated branch review

New Client API

c, err := dbc.NewClient(
    dbc.WithBaseURL("https://my-registry.example.com"),
    dbc.WithHTTPClient(myHTTPClient),
)
drivers, err := c.Search("adbc-driver-")
manifest, err := c.Install(cfg, "adbc-driver-postgresql")

Testing

All tests pass (go test ./...). Branch reviewed and passed by automated code review (27 commits, roborev job 1027).